1. Field of the Invention
The present invention relates to a motor drive device which converts AC power supplied from an AC power supply side into DC power, outputs the DC power, and then converts the DC power into AC power for driving a motor.
2. Description of the Related Art
Motor drive devices which drive motors in machine tools, press and forging machines, injection molding machines, industrial machines or various robots, convert AC power input from an AC power supply side into DC power, and then converts the DC power into AC power for use as power for driving the motor provided for each drive shaft. Such a motor drive device includes a converter which rectifies AC power supplied from an AC power supply side provided with a three-phase AC input power supply to output DC power, and an inverter which is connected to a DC link unit that is a DC output side of the converter and which mutually converts power between DC power of the DC link unit and AC power which is power for driving the motor or regenerated power, and controls velocity, torque or position of a rotator of the motor connected at an AC output side of the inverter. At the DC link unit which connects a DC side of the converter and a DC side of the inverter, a DC link capacitor is provided. The DC link capacitor functions as a smoothing capacitor for suppressing pulsation of a DC output of the converter, and is capable of accumulating DC power. The number of inverters provided is the same as the number of motors so that the inverters supply driving power respectively to the motors which are provided so as to correspond to a plurality of drive shafts to drive and control the motors, while the number of converters provided is usually one with respect to a plurality of inverters so as to reduce cost of the motor drive device and space.
In recent years, because there is a demand for energy conservation, power regenerative rectifiers which are capable of returning regenerated power generated upon deceleration of motors to an AC power supply side are commonly used as converters in motor drive devices. Such a rectifier converts AC power supplied from an AC power supply side into DC power to output to a DC side, and then converts the DC power supplied from the DC side into
AC power upon deceleration of a motor to output to the AC power supply side. Regenerated power generated at the motor upon deceleration of the motor is converted from AC power into DC power by an inverter, input to a converter via a DC link unit, further converted into AC power by the converter, and returned to the AC power supply side. At this time, voltage of a DC link capacitor of the DC link unit fluctuates according to the amount of the regenerated power generated at the motor and the amount of the AC power returned to the AC power supply side via the inverter, the DC link unit and the converter.
In a motor drive device which converts AC power supplied from an AC power supply side into DC power, outputs the DC power, and then converts the DC power into AC power for driving a motor to supply to the motor, a relationship among the amount of regenerated power generated at the motor, the amount of the AC power returned to the AC power supply side and a voltage of the DC link capacitor will be described below with reference to FIG. 5 to FIG. 8. FIG. 5 is a circuit diagram illustrating a flow of energy when the amount of the regenerated power generated at the motor is smaller than the amount of the AC power returned to the AC power supply side in the motor drive device, and FIG. 6 illustrates an increase of the voltage of the DC link capacitor in the case of FIG. 5. Further, FIG. 7 is a circuit diagram illustrating a flow of energy when the amount of the regenerated power generated at the motor is greater than the amount of the AC power returned to the AC power supply side in the motor drive device, and FIG. 8 illustrates an increase of the voltage of the DC link capacitor in the case of FIG. 7. A case will be described where, as illustrated in FIG. 5 and FIG. 7, the motor drive device 100 includes a converter 11 which rectifies AC power supplied from the AC power supply side provided with a three-phase AC input power supply 3 to output DC power, and a inverter 21 which is connected to the DC link unit 12 which is a DC output side of the converter 11 and which mutually converts power between DC power of the DC link unit 12 and AC power which is power for driving the motor 2 or regenerated power, and when the motor 2 which operates at constant speed by the motor drive device 100, is decelerated at time t1, regenerated power P1 is generated at the motor 2.
Diodes are connected in inverse parallel to the switching elements within the converter 11 so that the converter 11 can return the regenerated power generated upon deceleration of the motor to the AC power supply side. Because there is an upper limit in capability of the converter 11 to convert DC power into AC power, when the amount of regenerated power generated at the motor 2 exceeds the amount of power which can be converted from DC power into AC power by the converter 11, a voltage of the DC link capacitor C within the DC link unit 12 increases. The amount of AC power which can be returned to the AC power supply side is limited by the capability of the converter 11 to convert DC power into AC power. For example, as illustrated in FIG. 5, when the motor 2 which operates at constant speed, is decelerated at time t1, regenerated power P1 is generated at the motor 2. When the amount of this regenerated power P1 is within a range of the capability of the converter 11 to convert DC power into AC power, the voltage of the DC link capacitor C starts increasing at time t1 at which deceleration of the motor 2 starts by accumulation of the regenerated power P1 generated at the motor 2 in the DC link capacitor C as illustrated in FIG. 6, and the voltage of the DC link capacitor C becomes a constant value (at time t2) when the amount of power converted from DC power into AC power by the converter 11 (expressed by P2 in the drawing) is balanced with the amount of the regenerated power P1 generated at the motor 2. As illustrated in FIG. 7, when the regenerated power P1 generated when the motor 2 is decelerated at time t1 exceeds the capability of the converter 11 to convert DC power into AC power, the voltage of the DC link capacitor C starts increasing at time t1 at which the motor 2 starts deceleration by accumulation of the regenerated power P1 generated at the motor 2 in the DC link capacitor C as illustrated in FIG. 8, and because the amount of the regenerated power P1 generated at the motor 2 is larger than that can be converted by the conversion capability of the converter 11, the DC power continues to be accumulated in the DC link capacitor C, which results in continuing increase of the voltage of the DC link capacitor C.
The voltage of the DC link capacitor C, if exceeding a voltage of elements of the converter 11 and the inverter 21 or the DC link capacitor C itself, may lead to breakage of the elements. Therefore, generally, in the motor drive device, an “overvoltage alarm level” is set in advance for the voltage of the DC link capacitor C in the DC link unit 12 as a maximum voltage which is an allowable voltage in view of the withstand voltage of the elements of the converter 11 and the inverter 21 and the DC link capacitor C itself. The voltage of the DC link capacitor C in the DC link unit 12 is always monitored, and when the voltage exceeds the overvoltage alarm level, some kind of crisis prevention processing is executed.
For example, there exists a motor drive device in which, when the voltage of the DC link capacitor C in the DC link unit 12 exceeds the overvoltage alarm level, an overvoltage alarm signal is output to the inverter 21, and in response to this signal, the inverter 21 turns off switching operation for power conversion operation so as to stop conversion of the regenerated power generated at the motor 2 into the DC power (hereinafter, referred to as “stop conversion by alarm”). In this case, it is possible to block inflow of the regenerated power into the DC link unit 12, which suppresses increase of the voltage of the DC link capacitor.
Other than the above-described motor drive device which stops conversion by alarm, as a device that suppresses increase of the voltage of the DC link capacitor, there is for example, a motor drive device as disclosed in Japanese Unexamined Patent Publication No. 2008-271687 which additionally connects an auxiliary DC link capacitor to a DC link unit when a voltage of a DC link capacitor exceeds an overvoltage alarm level due to regenerated power generated at a motor. In this case, the regenerated power flowing into the DC link unit 12 is accumulated in the DC link capacitor with greater capacitance, so that it is possible to suppress increase of the voltage of the DC link capacitor.
In the above-described motor drive device which stops conversion by alarm, there is a time lag between time at which an overvoltage of the voltage of the DC link capacitor in the DC link unit is detected and time at which conversion operation of the inverter is actually stopped, due to generation and transmission time of the overvoltage alarm signal and time required for stop processing of the inverter. Depending on time from when the overvoltage of the voltage of the DC link capacitor is detected till when the conversion operation of the inverter is actually stopped, there is a possibility that although the overvoltage of the voltage of the DC link capacitor is detected, the voltage of the DC link capacitor will further increase and exceed the withstand voltage of the DC link capacitor, switching elements configuring the converter and the inverter, and elements such as diodes. Further, depending on the level of the regenerated power generated at the motor, the voltage of the DC link capacitor will further increase from when the overvoltage of the voltage of the DC link capacitors detected till when conversion operation of the inverter is actually stopped, and there is a possibility that the voltage of the DC link capacitor will exceed the withstand voltage of the DC link capacitor and the elements configuring the converter and the inverter.
Further, in either case of the above-described motor drive device which stops conversion by alarm and the motor drive device disclosed in Japanese Unexamined Patent Publication No. 2008-271687, the overvoltage alarm level is set lower than the withstand voltage to some extent while maintaining margin for security purpose taking into account the withstand voltage of the elements of the converter and the inverter and the DC link capacitor. Therefore, in some cases, such margin sometimes becomes an excessive margin.